This invention relates to a liquid developing method and apparatus for electrophotography which can develop a sheet of photosensitive material with direct electron injection system and to electrodes therefor.
FIG. 1 shows the structure of a prior art camera-type electrophotographic processor 1 schematically in cross section. In the figure, an original 3 held on an original table 2 is irradiated with light from a light source (not shown), the light is reflected from the surface of the original and a mirror 4 above, and converged by a lens 5. The focused light forms images on a sheet of electrophotographic paper (referred to simply as "master" hereinafter) P which is supplied from a paper feeding section 6 to a exposing section 7 by a conveying means. The exposed master P is conveyed by the conveying means to a developing section 8 for development, fixed at a fixing section 9, processed with etching solution at an etching section 10, dried and then taken out from an outlet 11.
The liquid developing apparatus and its operational principle used generaly at the developing section 8 of such camera-type electrophotographic processor 1 will be outlined below.
FIG. 2 shows a structure in cross section to show schematically a liquid developing apparatus 8A of "single jet system" disclosed in Japanese Patent Laid-open No. 185373/1984 which is most widely used because of a simple structure, low cost and easy maintenance. A main electrode 20 and an auxiliary electrode 21 comprising two electro-conductive members (referred to as "electrodes") applied forward or reverse bias voltage are provided, and a developing liquid discharge means 22 which discharges developing liquid DL is mounted between the main electrode 20 and the master P. A thread-like guide 23 made of nylon or the like is attached above the auxiliary electrode 21 in order to facilitate the passage of the master P as well as to prevent transfer of toner scum from auxiliary electrode to the back surface of the master P. Squeeze rollers 25 are provided further to squeeze out excessive developing liquid from the master P.
In the structure as above, while the master P is being moved over the thread-like guide 23 with the photosensitive surface (imaged surface) thereof facing upward, the charged toner particles within the developing liquid from the developing liquid discharge means 22 adhere to imaged parts on the master P in a predetermined amount depending on the intensity of electric field between the main electrode 20 and the master P to thereby conduct development.
The intensity of the electric field between the main electrode 20 and the master P varies depending on the state (relative dielectric constant) between the auxiliary electrode 21 and the master P, and when the developing liquid DL is filled to fully occupy the space therebetween, the electric field intensity increases to sufficiently attach the toner particles. However, when the air enters the space between the master P and the auxiliary electrode 21, the intensity of electric field is reduced. In a case where a large area on the master P is imaged with pictures or patterns, the toner particles are not adhered uniformly over the area to present irregular and non-uniform density.
As shown in FIG. 3, the master P comprises a base P2 made mainly of conductively processed paper and a photosensitive layer P1 coated thereon. For liquid electrophotographic development, the master P and the developing liquid DL are passed concurrently between the electrodes 20 and 21 which are opposed to each other in parallel at an interval of 1-3 mm. It is therefore necessary to fill not only the side of the photosensitive layer P1 but also the side of the base P2 with the developing liquid DL in order to develop a large area image. More particularly, as shown in FIG. 3, the reason why the base P2 should be filled with the developing liquid DL is because positive charges on the surface of the base P2 on backside of the photosensitive layer P1 is neutralized with negative charges supplied from the counter ion CN of the developing liquid DL. If the counter ion CN adheres on the surface of the base P2, it causes the positive charged toner particle PN to adhere on the side of the auxiliary electrode 21 simultaneously, and if the developing works continue, the many toner particles become adhered on the auxiliary electrode 21 to thereby deteriorate the performance.
It is heretofore necessary to clean the auxiliary electrodes periodically or to add an extra-cleaning apparatus. The maintenance becomes most difficult when the interval between electrodes is narrow and the size of the electrodes large. Especially when a large area image is developed, un-imaged background often becomes non-uniform foggy to lower the quality.